The present invention relates to a feed adjusting means for a sewing machine comprising a feed advance shaft on which is keyed an arm equipped with a guide bar. A slider which is in driving connection with a feed dog bar of the feed system is mounted on the guide bar.
Feed adjusting devices of the aforementioned type are already known in the art; these devices comprising a forked slider which is engaged with the slider and control means which are connected to the forked slider to move the slider along the guide bar and, when the latter is in motion, to vary the stroke of the feed dog bar and thus to vary the quantity of fabric advanced for each stitch formed by the sewing machine.
In the known devices, the forked slider is supported by a system of levers disposed so as to form substantially a parallelogram with the forked slider. However, this system tends to modify the form and position of the path of the slider as, during feed adjustment, the forked slider is displaced substantially parallel to itself along the guide bar which oscillates and thus the axis of the forked slider, along which the slider is displaced, moves even further from the optimum arc of the circle along which this slider would move if it were not retained by the forked slider and thus unless the slider is disposed remote from the axis of the feed advance shaft. During operation of the sewing machine, the displacement between the actual rectilinear path of the slider and the ideal path along the arc produces a continuous reciprocating movement of the slider along the guide bar; the amplitude of this reciprocating movement increasing in proportion to the aforementioned displacement.
The reciprocating movement produces excessive wear between the slider and the guide bar, vibrations which affect all the elements of the sewing machine and, in particular, those of the feed device, and also a substantial variation in the law of movement of the feed dog bar.
The object of the present invention is to reduce to a minimum the displacement between the actual path and the optimum path of the slider so as to eliminate any vibrations and avoid excessive wear to the elements of the feed device.
The technical problem to be solved in achieving this end is that of keeping the axis of the forked slider, and thus the actual path of the slider, coincident with the chord of this optimum path in every position of the slider with respect to the advance shaft.